Bandwidth provisioning for an entangled photon system

ABSTRACT

A quantum key distribution system is deployed in an optical fiber network transporting classical data traffic. A source of entangled photon pairs is used to generate quantum keys. Classical data traffic is typically transported over channels in the C-band. If a pair of channels for transport of quantum data is available within the C-band, then the source of entangled photon pairs is tuned to emit in a pair of channels in the C-band. If a pair of channels for transport of quantum data is not available within the C-band, then the source of entangled photon pairs is tuned to emit in a pair of channels in a combined S-band and L-band. When a periodically-poled lithium niobate waveguide pumped with a laser is used for the source of entangled photon pairs, the output spectral properties are tuned by varying the temperature of the waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior application Ser. No.15/944,389, filed on Apr. 3, 2018, which is a continuation of priorapplication Ser. No. 14/146,307, filed on Jan. 2, 2014, and issued asU.S. Pat. No. 9,967,637, which is a continuation of prior applicationSer. No. 13/592,504, filed Aug. 23, 2012, and issued as U.S. Pat. No.8,699,876, which is a continuation of prior application Ser. No.12/882,752, filed Sep. 15, 2010, and issued as U.S. Pat. No. 8,280,250,the disclosures of all are hereby incorporated by reference herein intheir entirety.

This application is related to U.S. patent application Ser. No.12/882,788, filed on Sep. 15, 2010, and issued as U.S. Pat. No.8,611,535, on Dec. 17, 2013, entitled “Characterization of an EntangledPhoton System”, the disclosure of which is herein incorporated byreference in its entirety.

BACKGROUND

The present disclosure relates generally to entangled photon systems,and more particularly to bandwidth provisioning of entangled photonsystems.

A method for providing secure transmission of data across a data networkinvolves encrypting the data at the source (sender), transmitting theencrypted data across the data network, and decrypting the encrypteddata at the receiver. Reliable methods for encryption/decryption includethose that use a secret key known only to the sender and receiver. Theissue then arises of how to transmit the key securely between the senderand the receiver.

Optical transmission across optical fibers is widely used intelecommunications networks. Quantum key distribution exploits thequantum physics properties of photons to securely transport keys acrossan optical network. One method of quantum key distribution encodesinformation bits in pairs of entangled photons. In each entangled pair,the quantum properties of the individual photons are strongly correlatedeven when they are separated geographically. In one architecture, asequence of pairs of entangled photons carrying the information bits forthe key are created at a centralized source. For each pair of entangledphotons, one photon is transmitted to User 1, and the correlated photonis transmitted to User 2. User 1 and User 2 can individually recover thekey from their respective sequence of received photons. Comparison ofthe quantum states of the photons received by each user can revealwhether a third party has eavesdropped on the quantum key transmissionor has substituted a separate quantum key.

In an optical network, channels can be used for quantum data orclassical data. Since the transport of quantum data uses very low photonfluxes, the transport of classical data strongly interferes with thetransport of quantum data. Method and apparatus for dynamicallyprovisioning quantum channels are advantageous for exploiting availablebandwidth.

BRIEF SUMMARY

A quantum key distribution system is deployed in an optical fibernetwork transporting classical data traffic. A source of entangledphoton pairs is used to generate quantum keys. Classical data traffic istypically transported over channels in a first frequency band. If a pairof channels for transport of quantum data is available within the firstfrequency band, then the source of entangled photon pairs is tuned toemit pairs of entangled photons in a pair of channels within the firstfrequency band. If a pair of channels for transport of quantum data isnot available within the first frequency band, then the source ofentangled photon pairs is tuned to emit pairs of entangled photons in apair of channels within a combined second frequency band and thirdfrequency band.

In an embodiment, the first frequency band is the telecommunicationsC-band, the second frequency band is the telecommunications S-band, andthe third frequency band is the telecommunications L-band. The source ofentangled photon pairs is a periodically-poled lithium niobate waveguidepumped with a laser. The output spectral properties of the light emittedfrom the waveguide is tuned by varying the temperature of the waveguide.

These and other advantages of the disclosure will be apparent to thoseof ordinary skill in the art by reference to the following detaileddescription and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a high-level schematic of an optical network configured todistribute quantum keys;

FIG. 1B shows a high-level schematic of an optical network configured totransport a combination of classical data and quantum data;

FIG. 2 shows a high-level schematic of an optical network configured tomeasure photon statistics;

FIG. 3 shows a high-level schematic of a source of photon pairs;

FIG. 4A and FIG. 4B show plots of output spectra as a function of thetemperature of the source of photon pairs;

FIG. 5A and FIG. 5B show plots of individual channels at the output of awavelength;

FIG. 6A-FIG. 6C shows plots of channel configurations for a wavelengthselective switch;

FIG. 7A and FIG. 7B show plots of coincidence probability as a functionof channel number;

FIG. 8 shows a flowchart of a method for provisioning quantum channels;

FIG. 9 shows a flowchart of a method for characterizing systemparameters of a quantum key distribution optical network;

FIG. 10A and FIG. 10B show plots of count probabilities as a function ofattenuation of pump power; and

FIG. 11 shows a high-level schematic of a computational systemimplementing a quantum key distribution control system.

DETAILED DESCRIPTION

FIG. 1A shows a high-level schematic of an embodiment of a quantum keydistribution (QKD) network implemented via the network of atelecommunications services provider. Shown are three principlelocations of interest: central office 102, fiber distribution plant 132,and customer premises 152. In the central office 102, a QKD server 104communicates with a source of photon pairs (SPP) 106. QKD server 104 cancommunicate with SPP 106 via different modes; for example, directly viaa local interface, remotely via a communications network, or via anintermediate control system, such as QKD control system 212 (describedbelow with reference to FIG. 2). QKD server 104 generates a randomnumber bit stream as a key. The random number bit stream modulatestransmission of photon pairs emitted by SPP 106. The two photons in apair are entangled with strong correlation of quantum states.

The output of SPP 106 is transmitted to the input port of a wavelengthselective switch (WSS) 108. The output ports of WSS 108 are coupled viaindividual optical fibers in the fiber distribution plant 132 to userequipment (UE) located in customer premises 152. Examples of UE includeservers and personal computers outfitted with, or coupled to,single-photon detectors (see below). In the example shown in FIG. 1A,there are six user equipment UE-A 154A-UE-F 154F, which in general aregeographically dispersed, coupled to WSS 108 via optical fiber134A-optical fiber 134F, respectively. The number of UE that can becoupled to the network depends on the number of output ports in WSS 108.For simplicity, each optical fiber is shown as a point-to-point runbetween an output port in WSS 108 and an input port in a UE. In general,there can be intermediate connections, such as in the local exchangeoffice, outside plant, and customer premises distribution closet (notshown).

In general, the locations of the network elements in FIG. 1A can beuser-specified and are not limited to the network of atelecommunications services provider. As mentioned above, QKD server 104can communicate remotely with SPP 106 via a communications network. QKDserver 104, SPP 106, WSS 108, and UE-A 154A-UE-F 154F can be located,for example, within a building, a campus, or a metro region. The fiberconnections can be provided over a private network instead of over thenetwork of a telecommunications services provider.

FIG. 2 shows a high-level schematic of an optical system fordistributing quantum keys. To simplify the description, an opticalsystem configured for a single pair of users is described. In general,the optical system can be configured for multiple pairs of users. Thesource of photon pairs (SPP) 202 transmits optical beam 251 from theoutput port 230 of SPP 202 over an optical fiber to input port 232 ofwavelength selective switch (WSS) 204. In this example, WSS 204 haseight output ports, referenced as output port A 234A-output port H 234H.To simplify the figure, only output port A 234A, output port B 234B, andoutput port H 234H are explicitly shown. Optical beam 253 is transmittedfrom output port A 234A of WSS 204 over an optical fiber to input port238 of single-photon detector 1 (SPD-1) 206. Similarly, optical beam 255is transmitted from output port B 234B of WSS 204 over an optical fiberto input port 240 of single-photon detector 2 (SPD-2) 208. Thesingle-photon detectors operate in a gated mode (discussed below) andare synchronized with an electro-optical modulator 304 (described belowwith reference to FIG. 3).

Coincidence counter 210 receives the detected signals from both SPD-1206 and SPD-2 208. In one embodiment, the single-photon detectors outputan electrical transistor-transistor logic (TTL) pulse for each detectionevent, and coincidence counter 210 measures both individual andcoincidence events. A coincidence event occurs when SPD-1 206 and SPD-2208 each detect a photon at the same time. In one embodiment,coincidence counter 210 can be implemented by a detector based on logicgates and implemented via a field programmable gate array (FPGA). Acoincidence counter can also be implemented by other electronicassemblies; for example, nuclear instrumentation modules (NIMs) and timedelay modules. In one embodiment, QKD control system 212 controls theoperation of the SPP 202, WSS 204, SPD-1 206, SPD-2 208, and coincidencecounter 210. An embodiment of QKD control system 212 is described belowwith reference to FIG. 11.

In other embodiments, the coincidence counts are not measured directlywith a coincidence counter. The clocks controlling the timing of SPD-1206 and SPD-2 208 are first synchronized. The individual counts fromSPD-1 206 and SPD-2 208 are then compared, and the coincident counts aredetermined.

FIG. 3 shows a high-level schematic of an embodiment of SPP 202 thatgenerates pairs of photons in the telcom frequency band by spontaneousparametric down conversion (SPDC) in a periodically-poled lithiumniobate (PPLN) waveguide. Pump laser 302, operating in a continuous-wave(CW) mode, transmits optical beam 341 to electro-optical modulator 304.A pump laser can also be operated in a pulsed mode. Pump laser 302 is asemiconductor Fabry-Perot (FP) laser. Other pump lasers can be used. Thewavelength of the optical beam 341 is tuned by controlling thetemperature of pump laser 302. In this example, the wavelength is tunedto λ=774.66 nm to center the down-converted spectrum on 1549.32 nm. Pumplaser 302 operates in a quasi-single-mode regime, in which the dominantmode is about 24.5 dB stronger than its nearest neighbors.Electro-optical modulator 304 is a lithium niobate electro-opticalmodulator driven by clock pulses (at a frequency of 1 MHz) from clockgenerator 320. An active feedback bias control (not shown) maintains anextinction ratio of 23 dB.

The output of electro-optical modulator 304 is modulated optical beam343, which is transmitted to attenuator (ATT) 306. The attenuation valueATT of attenuator 306 can be varied. The output of attenuator 306 isoptical beam 345, which is transmitted to PPLN waveguide 308. Inresponse to the pump laser light at a wavelength of λ=774.66 nm (opticalbeam 345), PPLN waveguide 308 emits photon pairs at a wavelength ofλ=1549.32 nm. The maximum power conversion efficiency of PPLN waveguide308 is 1.6×10⁻⁶ (measured from fiber pigtail to fiber pigtail, includingboth input and output fiber coupling losses). This efficiency remainsabove 0.5×10⁻⁶ for a wide temperature range from approximately 55° C. toapproximately 61° C. This efficiency corresponds to about 0.5 photonpairs over the entire spectrum per duration of the detector gate time(discussed below); the value is measured for zero value of attenuationATT in FIG. 3. The output of PPLN waveguide 308 at output port 330 isoptical beam 347, which includes the photon pairs at a wavelength ofλ=1549.32 nm and pump laser light at a wavelength of λ=774.66 nm.

Optical beam 347 is transmitted to blocker 310, which blocks the pumplaser light at a wavelength of λ=774.66 nm. In one embodiment, blocker310 is a wavelength division multiplex (WDM) coupler with greater than90 dB rejection of λ=774.66 nm light. Other filters can be used to blockthe pump laser light. The output of blocker 310 at output port 332 isoptical beam 349, which transmits the photon pairs at a wavelength ofλ=1549.32 nm. In practice, SPP 202 is housed in a transmitter. Theoutput port 332 of blocker 310 is coupled to the output port 230 of SPP202 via a short optical fiber. The output of output port 230 is opticalbeam 251 (see FIG. 2). Pump laser 302, electro-optical modulator 304,clock generator 320, attenuator 306, and PPLN waveguide 308 arecontrolled by SPP control system 322, which communicates with theoverall QKD control system 212 (see FIG. 2).

The spectral transmission window through an optical fiber is partitionedinto optical bands defined by wavelength ranges (or their correspondingfrequency ranges). Three bands of interest for telecommunications areL-band (1565-1625 nm), C-band (1530-1565 nm), and S-band (1460-1530 nm).Each band, furthermore, can be partitioned into channels; each channelis specified by a center wavelength or frequency and a channel width.The International Telecommunications Union (ITU) has developed a set ofindustry standards referred to as ITU grids which specify the set ofcenter frequencies. For dense wavelength division multiplexing (DWDM), agrid with a channel spacing of 100 GHz is commonly used.

A set of down-converted spectra emitted by SPP 202 is shown in FIG. 4Aand FIG. 4B. The horizontal axis 402 represents the frequency expressedas the difference Δf from a reference frequency, which, in thisinstance, is 193.5 THz. The vertical axis 404 represents the opticalpower in dBm. The output spectra can be tuned by varying the temperatureT of PPLN waveguide 308 or by varying the pump wavelength of opticalbeam 341 emitted by pump laser 302. FIG. 4A shows a family of spectra asthe temperature of the PPLN waveguide 308 is varied from T=54.0° C. toT=60.5° C. Two spectra, spectrum 412 and spectrum 414, are highlighted.For clarity, these two spectra are plotted separately in FIG. 4B.

Spectrum 412, obtained with a temperature T=56° C., has a singlespectral lobe with maximum power primarily over the C-band. Spectrum 412can be filtered by a bandpass filter with a bandwidth of 4.5 THz toobtain spectrum 418, which is fully confined within the C-band.

Spectrum 414, obtained with a temperature of T=60° C., has two spectrallobes: spectral lobe 414-S in the S-band and spectral lobe 414-L in theL-band. Wavelength selective switches are designed for adding/droppingchannels to/from WDM data streams. To accomplish this, the WSSpartitions the input spectrum into WDM channels and directs eachindividual channel to a specific output port (see, for example, WSS 204in FIG. 2). Groups of individual channels can be directed to the sameoutput port. In one embodiment, the WSS 204 has 45 WDM channelsconfigured on the 100 GHz-spaced ITU grid ranging from 191.6 THz to196.0 THz; each channel can be directed to any one of 8 output ports,output port A 234A-output port H 234H. Wavelength selective switcheswith different numbers of output ports are available.

FIG. 5A shows plots of loss (in dBm) as a function of frequency(expressed as Δf). In an embodiment, the dispersive element used in theWSS is an arrayed waveguide grating (AWG), which yields groups ofchannels in three output bands, referenced as C-band channel group 512,L-band channel group 510, and S-band channel group 514. The centerfrequencies of the output bands are separated by 6.79 THz, the freespectral range (FSR) of the grating. FIG. 5B shows a more detailed viewof C-band channel group 512. Shown are the transmission spectra of 38channels (19 pairs). Each channel has a nearly flat top with a 3 dBbandwidth of 77 GHz.

FIG. 6A-FIG. 6C show examples of three different WSS configurations, inwhich one channel (N_(ch) ^(A)) directed to output port A 234A, and theother channel (N_(ch) ^(B)=−N_(ch) ^(A)) is directed to output port B234B. For the C-band, the following convention is used for channelnumbering: N_(ch)(f)=10×(f−193.5), where f is the center frequency ofthe channel in THz. In FIG. 6A, channel 621A, channel 623A, and channel625A correspond to N_(ch) ^(A)=2, and channel 621B, channel 623B, andchannel 625B correspond to N_(ch) ^(B)=−2, respectively. In FIG. 6B,channel 631A, channel 633A, and channel 635A correspond to N_(ch)^(A)=10, and channel 631B, channel 633B, and channel 635B correspond toN_(ch) ^(B)=−10, respectively. In FIG. 6C, channel 641A, channel 643A,and channel 645A correspond to N_(ch) ^(A)=19, and channel 641B, channel643B, and channel 645B correspond to N_(ch) ^(B)=−19, respectively.

Single-photon detectors for wavelengths used in opticaltelecommunications are typically based on an avalanche photodiode (APD)operated in a counter mode. Operation of an APD is a function of thereverse bias voltage applied across the APD. When the bias voltageexceeds the breakdown voltage, an incident photon can initiate a carrieravalanche, resulting in a charge pulse that can be electronicallydetected. A carrier avalanche can also be initiated by a source otherthan an incident photon (such as a trapped charge or a phonon). Theseother sources can generate background counts, resulting in backgroundnoise.

To reduce background noise, an APD can be operated in a gated mode. Thebias voltage is modulated by a periodic sequence of pulses (such asrectangular pulses), referred to as bias pulses (also referred to asgate pulses). The amplitude of the bias pulses ranges from a pulsebaseline voltage less than the breakdown voltage to a pulse peak voltagegreater than the breakdown voltage. The pulse width is referred to asthe gate window (also referred to as the gate time). The inverse of theperiod between pulses is referred to as the trigger rate.

During a bias pulse, the bias voltage is greater than the breakdownvoltage, and an incident photon can initiate a carrier avalanche thatresults in a charge pulse that can be electronically detected. During anafterpulse interval (interval between two pulses), the bias voltage isless than the bias voltage. The charge state of the APD, however, doesnot relax instantaneously. During an afterpulse interval, an incidentphoton can still trigger a carrier avalanche, resulting in a detectedsignal. In addition, as discussed above, even in the absence of incidentphotons (dark conditions), other sources can trigger carrier avalanches,resulting in dark counts. Dark counts are dependent on the temperatureof the APD.

In the absence of dark counts, the probability of coincident detectionof two transmitted photons P₁₂ ⁰ and the probability of detecting atransmitted photon in the i-th detector P_(i) ⁰, (i=1, 2), depend on thephoton pair statistics. For the embodiment shown in FIG. 2 and FIG. 3,the statistics are Poissonian, and the following expressions for theprobabilities of interest can be derived:P ₁₂ ⁰=1−exp(−μQ ₁ T ₁η₁)−exp(−μQ ₂ T ₂η₂)+exp[−μ(Q ₁ T ₁η₁ +Q ₂ T ₂η₂−Q ₁₂ T ₁ T ₂η₁η₂)]  (E1)P _(i) ⁰=1−exp(−μQ _(i) T _(i)η_(i)).  (E2)

Here μ is the total average number of photon pairs over the entirespectrum (emitted by the source of photon pairs) per gate time, η_(i) isthe efficiency of the i-th detector, and T_(i) is thefrequency-independent transmittance of the optical path between the paircreation and photon detection points excluding the transmittance throughthe WSS itself. In this instance, T_(i) mostly reflects the outputfiber-coupling loss of the PPLN waveguide.

The quantities Q₁, Q₂ and Q₁₂ are introduced here to account forcreation and routing of a photon in a certain frequency band. Theprobability density function

(ω) for a photon pair to be generated at the offset frequencies ±ω fromone half of the pump frequency is the properly normalized down-convertedspectrum S(ω):

(ω)=2S(ω)/∫_(−∞) ^(+∞) S(ω)dω.  (E3)The overall action of the WSS can be described by the transfer functionsH_(p1)(ω) and H_(p2)(ω) relating the spectrum at the WSS input port 232to the output spectra at WSS output port A 234A (referred to here asport p1) and output port B 234B (referred to here as port p2). Thesefunctions reflect a particular configuration of the WSS and change everytime the WSS is reconfigured. Since conventional WSSs do not have abroadcasting capability, the two transfer functions do not overlap(H_(p1)(ω)H_(p2) (ω)=0). The joint probability that each photon of thesame pair is transmitted to the corresponding WSS ports is:Q ₁₂=∫_(−∞) ^(+∞)

(ω)|H _(p1)(ω)|² |H _(p2)(−ω)|² dω.  (E4)and the probability of a photon to appear at the i-th output port (wherei=1 corresponds to output port A 234A and i=2 corresponds to output portB 234B) is given by:Q _(i)=∫_(−∞) ^(+∞)

(ω)|H _(p1)(ω)|² dω.  (E5)

In the presence of dark counts, coincident detections at thesingle-photon detectors can arise from a photon pair generated in thePPLN and successfully transmitted through the fibers, from dark counts,or from a combination of the two. The total probability of a coincidencecan be calculated as the complement to one of the total probability ofseveral events. Using P₁₂ ⁰ and P_(i) ⁰ from (E1) and (E2) andintroducing P_(dci) as the probability of a dark count in the i-th SPD,the following expressions are obtained:

$\begin{matrix}{P_{12} = {1 - {\left( {1 - P_{{dc}\; 1}} \right)\left( {1 - P_{{dc}\; 2}} \right)\left( {1 - P_{12}^{0}} \right)} - {{P_{{dc}\; 1}\left( {1 - P_{{dc}\; 2}} \right)}\left( {1 - P_{1}^{0}} \right)} - {{P_{{dc}\; 2}\left( {1 - P_{{dc}\; 1}} \right)}\left( {1 - P_{2}^{0}} \right)}}} & ({E6}) \\{\mspace{79mu}{P_{i} = {1 - {\left( {1 - P_{dci}} \right){{\exp\left( {{- \mu}\; Q_{i}T_{i}\;\eta_{i}} \right)}.}}}}} & ({E7})\end{matrix}$

The above analysis above applies for other optical systems in which theloss of a quantum channel is frequency dependent. A closed formexpression for P₁₂ ⁰ exists even when the statistics of the pairs ismany-fold thermal. It converges to the expression of (E1) as the numberof modes goes to infinity.

The above analysis is first verified by measuring coincident countsbetween individual WDM channels in the C-band. The temperature of thePPLN waveguide 308 (see FIG. 3) is set to T=56° C., and the output isfiltered to ensure that the downconverted spectrum has no overlap withthe S-band and L-band (spectrum 418 in FIG. 4B). A pair of symmetricchannels, N_(ch) ^(A) and N_(ch) ^(B)=N_(ch) ^(A), is directed to port A234A and port B 234B (see FIG. 2), respectively. The coincidence countsare measured for 30 seconds.

In the plot shown in FIG. 7A, the horizontal axis 702 represents thechannel number N_(ch) ^(A); the vertical axis 704 represents thecoincidence counts P₁₂. In FIG. 7A, the individual data points 712 areplotted. The dependence is not flat, mostly due to the wavelengthdependent loss of the WSS 204 (see FIG. 2). The independent measurementsof the loss spectrum permit evaluation of the integrals Q₁, Q₂, and Q₁₂.Using them together with the independently obtained maximum averagenumber of pairs per unit time μ_(max) (see discussion below for moredetails of this parameter), the coincidence probability P₁₂ can becalculated from (E6). The calculated values are shown as curve 714 inFIG. 7A. Note that the calculated curve fits the data well and capturesnearly all features of a non-trivial frequency profile exhibited by thedata.

The above analysis can be further verified with channels in the S-bandand L-band. The test utilizes the free spectral range (FSR) periodicityof a WSS. For the same configuration of the WSS described above (N_(ch)^(A) and N_(ch) ^(B)=−N_(ch) ^(A)), the actual frequency bands that aredirected to each port include two well-separated WDM peaks, one in theS-band and one in the L-band. The aggregate outputs are not completelysymmetric in frequency; also, they vary in loss. The center frequenciesof the three bands (S, C, and L) are separated by exactly the sameamount FSR=6.79 THz, but the channel separation within each band isdifferent for each band. In WSS 204, the channel separations within theS-band, C-band, and L-band are 103.6 GHz, 99.9 GHz, and 96.5 GHz,respectively. This behavior arises from the material properties and iscommon for all AWGs, on which WSSs are based. The specific values of thechannel separations are dependent of the particular WSS component used.

For large channel numbers (|N_(ch) ^(A,B)|≥10), the symmetry is lostand, therefore, the coincidence counts drop, as shown in FIG. 7B.Plotted in FIG. 7B are the data points 722. Curve 724, calculated from(E6), is in good agreement with the data. Because the coincidence rateremains relatively high for at least a few central channels, the FSRperiodicity property of AWGs, together with the temperature tuning ofthe PPLN waveguide, can be used to quickly move the quantum channel toand from the C-band without interrupting QKD service for a long periodof time.

The above spectral properties can be used for bandwidth provisioning ofa QKD system. In optical fiber systems, typically only the C-band isused. The loss in the C-band is lower than in the S-band or L-band, andmore channels are available in the C-band than in the S-band or L-band.The term “classical channel” refers to a channel carrying conventionaldata, in distinction to a “quantum channel” carrying quantuminformation. Since quantum channels utilize low photon fluxes,conventional data (classical data) transported over classical channelscan strongly interfere with quantum information (quantum data)transported over quantum channels in the same band. If classical datatraffic is not too heavy, there can be suitable pairs of channels in theC-band available for quantum data traffic. If classical data traffic isheavy and there are no suitable pairs of channels available in theC-band for quantum data traffic, then quantum data traffic can bedynamically allocated to a pair of channels utilizing a combination ofthe S-band and L-band.

In the optical network previously shown in FIG. 1A, a single opticalsource (source of photon pairs 106) is coupled to the input port of WSS108. Other optical sources, including sources of photon pairs forquantum channels and conventional optical sources (such as lasers) forclassical channels, can be coupled together and directed to the inputport of WSS 108. In the optical network shown in FIG. 1B, there are fourinput optical sources: laser 110, laser 114, source of photon pairs 112,and the previous source of photon pairs 106. The output from the fourinput optical sources are fed into input ports of combiner/multiplexer116. The output optical beam 101 from combiner/multiplexer 116 istransmitted into the input port 108I of WSS 108. QKD control system 212is depicted connected to input port 108I of WWS 108, source of photonpairs 106, and source of photon pairs 112.

Optical beam 103 is transmitted from output port 108A of WSS 108, acrossoptical fiber 134A, and into the input port of demultiplexer 156. One ofthe output ports of demultiplexer 156 is coupled to the input port ofUE-A 154A. Similarly, optical beam 105 is transmitted from output port108B of WSS 108, across optical fiber 134B, and into the input port ofdemultiplexer 158. One of the output ports of demultiplexer 158 iscoupled to the input port of UE-B 154B. Light from the input opticalsources can be switched to UE-A 154A or UE-B 154B by controlling thewavelengths of the input optical sources and the configuration of WSS108.

A flowchart of a method for bandwidth provisioning is shown in FIG. 8.In step 802, classical data traffic on C-band channels is monitored atone or more reference points in the optical network. Suitable referencepoints include input port 108I, output port 108A, and output port 108Bof WSS 108. Other suitable reference points are points along opticalfibers coupled to these ports. The process then passes to decision step804. If a suitable pair of quantum channels is available in the C-band(determined by the relationship: the frequency of the first channel+thefrequency of the second channel=2× the pump laser frequency), then theprocess passes to step 806, and the temperature of PPLN waveguide 308 isadjusted to a first temperature T₁ (in one embodiment, T₁ isapproximately 56° C.) to produce a single-lobe spectrum in the C-band.The process then passes to step 808, in which a pair of entangledphotons is transmitted across a pair of quantum channels in the C-band.

Refer back to decision step 804. If a suitable pair of quantum channelsin the C-band is not available, then the process passes to step 810, andthe temperature of PPLN waveguide 308 is adjusted to a secondtemperature T₂ (in one embodiment, T₂ is approximately 60° C.) toproduce a dual-lobe spectrum, with one lobe in the S-band and one lobein the L-band. The process then passes to step 812, in which a pair ofentangled photons is transmitted across a pair of quantum channels inthe combined S-band and L-band.

In the embodiment described above, the spectral properties of the lightemitted by PPLN waveguide 308 are tuned by varying the temperature ofthe PPLN waveguide 308. In another embodiment, the spectral propertiesof the light emitted by PPLN waveguide 308 are tuned by varying thefrequency of the pump laser light emitted by pump laser 302 (see FIG.3). One skilled in the art can develop other embodiments in which asource of entangled photon pairs can be tuned to emit pairs of entangledphotons in either a first frequency band or in a combination of a secondfrequency band and a third frequency band.

Monitoring the key performance parameters of a QKD system is importantfor reliable operation. Measurements that can be performed withoutdisconnecting network elements are advantageous to minimize systemdowntime. Refer to FIG. 2 and FIG. 3. For a QKD system with a source ofentangled photon pairs, three primary operational system parameters of aQKD system are the efficiency η₁ of SPD-1 206, efficiency η₂ of SPD-2208, and the maximum average number of photon pairs per unit timeμ_(max) at the output port 330 of the PPLN waveguide 308. Note that theaverage number of photon pairs μ emitted by PPLN waveguide 308 is afunction of the input pump power. Here μ_(max) refers to the maximumaverage number of photon pairs per unit time generated by PPLN waveguide308 at maximum input pump power.

Refer to FIG. 2 and FIG. 3. As discussed above, PPLN waveguide 308typically is packaged in a transmitter; therefore, output port 330 isnot directly accessible by the telecommunications services provider. Theclosest access port will vary with the system configuration. In FIG. 3,the closest access port is output port 230 of SPP 202. In otherinstances, output port 230 is connected via a fiberoptic cable to anoptical connector (or splitter) on a connector panel (not shown), andservice access is provided at the connector panel. In general, therewill be loss between output port 330 and the service access port (which,for this discussion, is assumed to be output port 230). Thisport-to-port loss loss_(pt-pt) can be about a few dB.

This loss can result in broken pairs of photons. If pairs of photons areemitted at output port 330, then, at output port 230, there can be acombination of pairs of photons as well as single unpaired photons. Thesingle unpaired photons cannot be used for quantum key distribution. Ifthe maximum average number of photon pairs per unit time emitted at theoutput port 330 is μ_(max), then the average number of photons per unittime measured at output port 230 is 2μ_(max)T_(pt-pt), where T_(pt-pt)is the transmittance corresponding to loss_(pt-pt). At output port 230,the average number of photon pairs per unit time is μ_(max)T_(pt-pt) ²and the average number of single unpaired photons per unit time is2μ_(max)T_(pt-pt)(1−T_(pt-pt)). Since T_(pt-pt), in general, is notknown to the service provider, the average number of photon pairs perunit time μ_(max)T_(pt-pt) ² at output port 230 cannot be directlyderived from measurements of 2μ_(max)T_(pt-pt). To maintain highsecurity, the service provider needs to adjust the average number ofphoton pairs per unit time as a function of various network parameters,such as the distances between the source and the detectors and thebandwidth of the links between the source and the detectors.

FIG. 9 shows a flowchart of steps for a method for measuring theoperational systems parameters of a QKD system. In step 902, the lossloss₁ between port 330 of SSP 202 and port 238 of SPD-1 206 and the lossloss₂ between port 330 of SSP 202 and port 240 of SPD-2 208 aremeasured. The process then passes to step 904, in which thetransmittance

$T_{1} = 10^{\frac{{loss}_{1}{({dB})}}{10}}$and the transmittance

$T_{2} = 10^{\frac{{loss}_{2}{({dB})}}{10}}$are calculated. The process then passes to step 906, in which thetrigger rate R of SPD-1 206 and SPD-2 208 is set. The process thenpasses to step 908, in which the pump laser 302 is turned off. Theprocess then passes to step 910, in which the dark count D_(dark1) atSPD-1 206 and the dark count D_(dark2) at SPD-2 208 are recorded over apredetermined time interval (for example, 1 sec). The process thenpasses to step 912, in which the dark count probabilityP_(dc1)=D_(dark1)/R and the dark count probability P_(dc2)=D_(dark2)/Rare computed.

The process then passes to step 914, in which the pump laser 302 isturned on. The process then passes to step 916, in which the pump powerinto the PPLN waveguide 308 is adjusted by varying the attenuation ATT(here ATT<0) of the attenuator 306. The process then passes to step 918,in which the detector count D_(count1) at SPD 1 206, the detector countD_(count2) at SPD 2 208, and the coincidence count D_(coin) are recordedover a predetermined time interval. The process then passes to step 920,in which the count probability P₁=D_(count1)/R, the count probabilityP₂=D_(count2)/R, and the coincidence count probability P₁₂=D_(coin)/Rare computed. The process then passes to decision step 922. Step916-step 920 are to be iterated for a total of N (a predeterminedinteger) different values of the output power of PPLN waveguide 308 byadjusting the attenuation ATT to vary the input pump power. Ifmeasurements are to be taken at a new output power level of PPLNwaveguide 308, then the process returns to step 916, in which the outputpower is adjusted to a new level; step 918 and step 920 are thenrepeated.

When N iterations have been completed, the process then passes from step922 to step 924, in which the operational systems parameters arecomputed. The data collected from the multiple iterations of step916-step 920 yields the three experimentally determined functionsP₁(ATT), P₂(ATT), and P₁₂(ATT). A joint fit of the three experimentallydetermined functions are performed with the following three analyticalfunctions, in which μ_(max) (maximum average number of photon pairs perunit time), η₁ (efficiency of SPD-1 206) and η₂ (efficiency of SPD-2208) are the fitting parameters:

$\begin{matrix}{\mspace{79mu}{P_{1} = {1 - {\left( {1 - P_{{dc}\; 1}} \right) \times {\exp\left( {{- 10^{\frac{ATT}{10}}} \times \mu_{\max} \times T_{1} \times \eta_{1}} \right)}}}}} & ({E8}) \\{\mspace{79mu}{P_{2} = {1 - {\left( {1 - P_{{dc}2}} \right) \times {\exp\left( {{- 10^{\frac{ATT}{10}}} \times \mu_{\max} \times T_{2} \times \eta_{2}} \right)}}}}} & ({E9}) \\{{P_{12} = {1 - {\left( {1 - P_{{dc}\; 1}} \right) \times \left( {1 - P_{{dc}\; 2}} \right) \times \left( {1 - P_{12}^{0}} \right)} - {P_{{dc}\; 1} \times \left( {1 - P_{{dc}\; 2}} \right) \times \left( {1 - P_{2}} \right)} - {P_{{dc}\; 2} \times \left( {1 - P_{{dc}\; 1}} \right) \times {\left( {1 - P_{1}} \right).\mspace{79mu}{where}}}}},} & ({E10}) \\{P_{12}^{0} = {1 - {\exp\left( {{- 10^{\frac{ATT}{10}}} \times \mu_{\max} \times T_{1} \times \eta_{1}} \right)} - {\exp\left( {{- 10^{\frac{ATT}{10}}} \times \mu_{\max} \times T_{2} \times \eta_{2}} \right)} + {\exp\left( {{- 10^{\frac{ATT}{10}}} \times \mu_{\max} \times \left( {{T_{1} \times \eta_{1}} + {T_{2} \times \eta_{2}} - {T_{1} \times \eta_{1} \times T_{2} \times \eta_{2}}} \right)} \right)}}} & ({E11})\end{matrix}$Using standard curve-fitting techniques, the best joint fits yield theoperational systems parameters μ_(max), η₁ and η₂. Relationships(E8)-(E11) hold for a frequency-selective splitter such as a wavelengthdivision demultiplexer, wavelength selective switch, and areconfigurable optical add/drop multiplexer. For other splitters, suchas a 1:N splitter (where N is the number of output ports) with nowavelength demultiplexing, other relationships can be derived.

Examples of results are shown in FIG. 10A and FIG. 10B. Values ofattenuation ATT are plotted along the horizontal axis 1002. The valuesof P₁(ATT) and P₂(ATT) are plotted along the vertical axis 1004 in FIG.10A; the values of P₁₂(ATT) are plotted along the vertical axis 1024 inFIG. 10B. In FIG. 10A, data points 1006 represent the measured values ofP₁(ATT), and data points 1008 represent the measured values of P₂(ATT).Plot 1010 represents the best-fit curve from (E8), and plot 1012represents the best-fit curve from (E9). In FIG. 10B, data points 1026represent the measured values of P₂(ATT), and plot 1030 represents thebest-fit curve from (E10). In this example, the derived values of theoperational systems parameters are η₁=10%, η₂=13%, and μ_(max)=0.57.

FIG. 11 shows a schematic of an embodiment of a computational system forimplementing a QKD control system 212 (see FIG. 2). One skilled in theart can construct the computational system 1102 from variouscombinations of hardware, firmware, and software. One skilled in the artcan construct the computational system 1102 from various combinations ofelectronic components, such as general purpose microprocessors, digitalsignal processors (DSPs), application-specific integrated circuits(ASICs), and field-programmable gate arrays (FPGAs).

Computational system 1102 comprises computer 1104, which includes acentral processing unit (CPU) 1106, memory 1108, and data storage device1110. Data storage device 1110 comprises at least one non-transitory,persistent, tangible computer readable medium, such as non-volatilesemiconductor memory, a magnetic hard drive, and a compact disc readonly memory.

Computational system 1102 can further comprise one or more userinput/output interfaces that interface computer 1104 with userinput/output devices. For example, user input/output interface 1120interfaces computer 1104 with user input/output device 1140. Examples ofuser input/output device 1140 include a keyboard, a mouse, and a localaccess terminal. Data, including computer executable code, can betransferred to and from computer 1104 via user input/output interface1120. Computational system 1102 can further comprise a video displayinterface (not shown), which drives a video display (not shown) and canreceive user input from the video display.

Computational system 1102 can further comprise one or moreinstrumentation interfaces. For example, instrumentation interface A1122 interfaces computer 1104 with source of photon pairs 202 (see FIG.2); instrumentation interface B 1124 interfaces computer 1104 withwavelength selective switch 204; instrumentation interface C 1126interfaces computer 1104 with single-photon detector 1 206;instrumentation interface D 1128 interfaces computer 1104 withsingle-photon detector 2 208; and instrumentation interface E 1130interfaces computer 1104 with coincidence counter 210.

Computational system 1102 can further comprise one or morecommunications network interfaces that interface computer 1104 withcommunications networks, such as local area networks and wide areanetworks. Data, including computer executable code, can be transferredto and from computer 1104 via communications network interfaces. Forexample, communications network interface 1132 interfaces computer 1104with communications network 1152. Computational system 1102 can beremotely accessed and controlled via communications network 1152. A QKDserver (for example, QKD server 104 in FIG. 1) can communicate withcomputational system 1102 via communications network 1152 or via a localinterface (not shown). In some embodiments, source of photon pairs 202,wavelength selective switch 204, single-photon detector 1 206,single-photon detector 2 208, and coincidence counter 210, eitherindividually or in combination, can communicate with computer 1104 viacommunications network 1152.

As is well known, a computer operates under control of computersoftware, which defines the overall operation of the computer andapplications. CPU 1106 controls the overall operation of the computerand applications by executing computer program instructions that definethe overall operation and applications. The computer programinstructions can be stored in data storage device 1110 and loaded intomemory 1108 when execution of the program instructions is desired. Themethod steps shown in the flowcharts in FIG. 8 and FIG. 9 can be definedby computer program instructions stored in memory 1108 or in datastorage device 1110 (or in a combination of memory 1108 and data storagedevice 1110) and controlled by the CPU 1106 executing the computerprogram instructions. For example, the computer program instructions canbe implemented as computer executable code programmed by one skilled inthe art to perform algorithms implementing the method steps shown in theflowcharts in FIG. 8 and FIG. 9. Accordingly, by executing the computerprogram instructions, the CPU 1106 executes algorithms implementing themethod steps shown in the flowcharts in FIG. 8 and FIG. 9.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the inventive concept disclosed herein is not to be determined fromthe Detailed Description, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present disclosure and thatvarious modifications may be implemented by those skilled in the artwithout departing from the scope and spirit of the disclosure. Thoseskilled in the art could implement various other feature combinationswithout departing from the scope and spirit of the disclosure.

The invention claimed is:
 1. A method comprising: determining whether apair of channels in an optical fiber network are available fortransmission of a pair of quantum entangled photons via a firstfrequency band associated with one of a plurality of optical sources,wherein the pair of channels is not available in the first frequencyband when a quantity of non-quantum data traffic in the first frequencyband is heavy; and transmitting the pair of quantum entangled photonsvia a second frequency band in the optical fiber network in response todetermining that the pair of channels is not available in the firstfrequency band.
 2. The method of claim 1, further comprising:determining, at a subsequent time period, that the pair of channels isavailable in the first frequency band; and transmitting a second pair ofquantum entangled photons via the first frequency band in response tothe determining that the pair of channels is available in the firstfrequency band at the subsequent time period.
 3. The method of claim 2,wherein the one of the plurality of optical sources comprises aperiodically-poled lithium niobate waveguide.
 4. The method of claim 3,wherein the transmitting the second pair of quantum entangled photonsvia the first frequency band comprises: adjusting a temperature of theperiodically-poled lithium niobate waveguide.
 5. The method of claim 4,wherein the temperature is 56 degrees Celsius.
 6. The method of claim 4,wherein the temperature is 60 degrees Celsius.
 7. The method of claim 1,wherein the first frequency band is a telecommunications C-band and thesecond frequency band is a telecommunications S-band.
 8. An apparatuscomprising: a memory storing computer program instructions; and aprocessor communicatively coupled to the memory, the processorconfigured to execute the computer program instructions, which, whenexecuted, cause the processor to perform operations, the operationscomprising: determining whether a pair of channels in an optical fibernetwork are available for transmission of a pair of quantum entangledphotons via a first frequency band associated with one of a plurality ofoptical sources, wherein the pair of channels is not available in thefirst frequency band when a quantity of non-quantum data traffic in thefirst frequency band is heavy; and transmitting the pair of quantumentangled photons via a second frequency band in the optical fibernetwork in response to determining that the pair of channels is notavailable in the first frequency band.
 9. The apparatus of claim 8, theoperations further comprising: determining, at a subsequent time period,that the pair of channels is available in the first frequency band; andtransmitting a second pair of quantum entangled photons via the firstfrequency band in response to the determining that the pair of channelsis available in the first frequency band at the subsequent time period.10. The apparatus of claim 9, wherein the one of the plurality ofoptical sources comprises a periodically-poled lithium niobatewaveguide.
 11. The apparatus of claim 10, wherein the transmitting thesecond pair of quantum entangled photons via the first frequency bandcomprises: adjusting a temperature of the periodically-poled lithiumniobate waveguide.
 12. The apparatus of claim 11, wherein thetemperature is 56 degrees Celsius.
 13. The apparatus of claim 11,wherein the temperature is 60 degrees Celsius.
 14. The apparatus ofclaim 8, wherein: the first frequency band is a telecommunicationsC-band; and the second frequency band is a telecommunications S-band.15. A computer readable medium storing computer program instructionswhich, when executed on a processor, cause the processor to performoperations, the operations comprising: determining whether a pair ofchannels in an optical fiber network are available for transmission of apair of quantum entangled photons via a first frequency band associatedwith one of a plurality of optical sources, wherein the pair of channelsis not available in the first frequency band when a quantity ofnon-quantum data traffic in the first frequency band is heavy; andtransmitting the pair of quantum entangled photons via a secondfrequency band in the optical fiber network in response to determiningthat the pair of channels is not available in the first frequency band.16. The computer readable medium of claim 15, the operations furthercomprising: determining, at a subsequent time period, that the pair ofchannels is available in the first frequency band; and transmitting asecond pair of quantum entangled photons via the first frequency band inresponse to the determining that the pair of channels is available inthe first frequency band at the subsequent time period.
 17. The computerreadable medium of claim 16, wherein the one of the plurality of opticalsources comprises a periodically-poled lithium niobate waveguide. 18.The computer readable medium of claim 17, wherein the transmitting thesecond pair of quantum entangled photons via the first frequency bandcomprises: adjusting a temperature of the periodically-poled lithiumniobate waveguide.
 19. The computer readable medium of claim 18, whereinthe temperature is 56 degrees Celsius.
 20. The computer readable mediumof claim 18, wherein the temperature is 60 degrees Celsius.